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Topic: Compression 80 psi (Read 450 times)

My 1999 bullet 500 has nearly 24000 miles on the clock and has begun to get hard to start, I checked the compression cold at it was aproximately 60 psi. I added a teaspoon of oil and compression increased to 80 psi. I also checked the compression hot, and at operating temperature, readings were 80 psi.

I might add that I am at an elevation of 5000 ft. I am assuming my rings are getting worn and are due for renewal. What are my options? Should I just replace the stock rings?, replace the stock piston?, with what? Will I need a cylinder rebore, what should I look for?

At 5000 feet elevation, your compression reading should probably be around 85-88 psi with everything normal.So, you are a bit down on compression, and it might be hard to get started with compression under 60 psi.

If you still have the stock piston in there, this might be a good time to get it out. You can count your blessings that it hasn't come apart on you yet.

My recommendation:Remove the head, decoke, and do a valve job. Pay close attention to the fit of the valves to the valve guides, and renew if necessary to get the proper clearance. Use valve stem seals if you can.Remove the barrel, get a piston in whatever larger bore size you prefer, and have the barrel properly bored/honed to fit. An optional upgrade would be to get an alloy barrel for better cooling. If you plan to further increase your power output, getting the alloy barrel would be mandatory.

24000 miles is a good mileage to be doing these things. Back in "the day" this was typical mileage for needing a top end job on a Brit bike.

It looks like a logarithmic curve starting at roughly 3% loss in pressure every 1000'. So at an elevation (5500'), I'd be looking at about a 15% loss.

So Ace, what would the procedure be to obtain some power but retain ease of starting? Would I pull the head and barrel, take it to a machine shop to see if it needs a rebore and then order a piston to fit that rebore? Or is that cost the same as ordering a new alloy cylinder and corresponding piston from the start?

It looks like a logarithmic curve starting at roughly 3% loss in pressure every 1000'. So at an elevation (5500'), I'd be looking at about a 15% loss.

So Ace, what would the procedure be to obtain some power but retain ease of starting? Would I pull the head and barrel, take it to a machine shop to see if it needs a rebore and then order a piston to fit that rebore? Or is that cost the same as ordering a new alloy cylinder and corresponding piston from the start?

My advice is the bore to a larger size like 535, and use a higher compression piston of high quality.Now, it might look unseemly for me to mention our stuff here on the forum, but the fact is that we have the lightest piston made anywhere for the Bullet. This puts the least stress on the lower end of the engine. Even less than a stock piston. Set the compression using gaskets/spacers under the barrel to something more like 110-120 psi on the cranking compression test, which would put you in the same neighborhood as a stock Bullet at sea level.

We can do the boring/honing to fit the piston correctly with a clearance which we know is correct, and we do it on custom-made torque plates which simulate the barrel being torqued on to the engine when we bore it, so that it is in the same "roundness" condition when you torque it on your engine(20 ft-lbs). Break it in right, which we can instruct you about.The bore is always honed to fit the exact piston that you have selected, with the proper clearance. Not the other way around. You fit the bore to the piston.

Make sure that your breather is the old type which has the elbow on the left side of the crankcase, and if it has the breather on the oil tank, then now is the time to convert it to the old style. It really needs that. And make sure it has a duckbill working on the end of the breather hose.

If you want valve job work or valve guides or whatever, we can do all that, and we only do performance "Mondello" valve jobs, so it will perform better than pretty much any other valve job you are going to get. We'll make guides to fit, if you need them. Any level of performance head work above and beyond that, we can also do for you if you want.